Grains such as corn have long been used as a source of starch. One of the classic methods of separating starch and other components from grain is the wet milling process. This method is a highly specific and integrated system developed to separate the major components of the kernel as completely as possible (see Stanley A. Watson, Starch: Chemistry & Technology, Vol. II, Industrial Aspects, Academic press, New York, 1967, pgs. 30-51. While the process in commercial practice is essentially a continuous one, it is convenient to break the process down into general component steps. The first step in the wet milling process is the preparation of the grain kernel by cleaning. The grain is screened to remove all large and small pieces of chaff, sand and other undesirable foreign material. Rust and light chaff are removed by aspiration. The next step in the wet milling of grain is the steeping. The grain must be softened by a steeping process to produce optimum separation of components. Steeping, however, is more than just soaking in water. It involves maintaining the correct balance of water flow, temperature, sulfur dioxide concentration, and an adequate population of lactic acid bacteria. It may also include a number of sequential soaks under various conditions. Grain is placed in a steep tank and steep water (usually drained mill water or other process water) is added along with desired additives. Steep water will cascade from tank to tank from oldest corn to progressively less steeped corn. At the end of the steep, the corn is removed from the tank, new corn added and the process is repeated. Corn, for example, is normally steeped 30 to 50 hours at a temperature of 48.degree. to 52.degree. C. By the end of the steeping period, the corn should have (a) absorbed water up to about 45% on a wet basis, (b) released about 6.0-6.5% of its dry substance or solubles into the steep water; (c) absorbed about 0.2 to 0.4 of sulfur dioxide per kilogram; and (d) become sufficiently soft to yield when squeezed between the fingers (see Watson et al., Cereal Chem., Vol. 28, pg. 105 (1951)). After this step essentially all of the steep water is separated from the steeped corn.
The final step is the wet milling and fraction separation which is designed to provide as complete a separation of the component parts of the kernal as possible and practical. The drained grain is added to a mill, along with process water. This milling step may actually comprise a plurality of grinding steps, such as an initial cracking step or coarse grinding to remove the germ followed by one or more increasingly finer grinds to remove additional components. These are essentially continuous grinds with water and grain constantly being added and constantly removed from each mill. In between grinds, the components can be separated by screening or washing. The removal of starch in the milling process is such that it must be separated from the fiber. Starch is normally separated with a combination of screens, centrifuges and wash water. It is often the case that the used washed water or process water is then recycled for use as the steep liquid.
In spite of this complicated extraction procedure, a percentage (around 2 to 6%) of the starch remains bound to the fiber, which is normally never recovered. This represents from about 20% to 60% of the weight of the fiber.
It has been known for some time that addition of cellulase could be beneficially added to the steep liquid (see R. Takahashi, T. Ojima, and K. Yoshimura, "Cereal Starch Production Using Cellulase," J. Ferment Technol., Vol. 44, No. 11, pgs. 842-846 (1966) 5-14 liters/ton with no mention of activity. See also Malmos, AIChE, No. 172, Vol. 74, pg. 95, (1978)), to increase starch yield or shorten processing time. As discussed in the prior art, the normal dosage added to steep liquid is around 0.1 to 0.2 liters per ton of grain. No mention of relative activity is discussed, however, which may account for the wide variety of results in the prior art. The steep liquid containing the unused or inactivated enzyme is usually extracted for solids or used as animal or nutrient feed. However, because of the high content of SO.sub.2 (about 2000 ppm) at the beginning of the steeping process, which creates a low pH and may inactivate the enzyme, it is usually recommended that no enzyme be added to the steep liquid until the SO.sub.2 content reaches 500 ppm or the pH is up around 4-4.5. (see e.g. FinnSugar sales literature for Spezyme CE brand cellulase; see also Takahashi, J. Ferment Tech, 44 (11), pg. 842-846 (1966)). This creates a problem. Because a number of tanks are used for sequential steeping, the actual tank to which enzyme must be added constantly changes. It is time consuming, complicated and expensive to continually move equipment and measure SO.sup.2 levels to meet these constant changes. Further, practical results outside of the laboratory for this process have not been demonstrated with any reliability. It appears that the enzyme does not actually penetrate the outer hull during steeping and as such does little to separate components. Cellulase appears to only work on inner layers and soluble glucans. Some positive results in the prior art for steeping can probably be explained because the prior art has added cellulase to preground grain in the steep tank rather than whole grain as actually practiced commercially, thus giving the cellulase access to the inner layers. In practice, however, unsteeped grain is never ground except in dry milling process. The grinding of unsteeped grain would decrease the recovery of the lighter components such as the germ. Further, it appears that steeping experiments have only been conducted on laboratory scale.
Accordingly, it would be advantageous to have a method of wet milling which overcomes the need to measure pH or SO.sub.2 concentration, requires less equipment or labor and be less complicated, yet still achieve increased separation of starch and other grain components. It would further be advantageous to be able to decrease the steep time without deleterious effect upon the separation products.